Inspection processes are used at various steps during semiconductor manufacturing to detect defects on wafers to increase yield. Inspection becomes more important to the successful manufacture of semiconductor devices as the dimensions of semiconductor devices decrease because smaller defects can cause the devices to fail. Semiconductor manufacturers seek improved sensitivity to particles, anomalies, and other defect types, while maintaining overall inspection speed (in wafers per hour) in wafer inspection systems.
The demands of the semiconductor industry for wafer and photomask inspection systems exhibiting high throughput and improvements in resolution are ongoing. Some inspection systems try to achieve higher resolution by illuminating the wafer or reticle using light having shorter wavelengths.
Generating light at wavelengths below 400 nm, and especially below 300 nm, can be challenging. Light sources used for semiconductor inspection require relatively high powers, long lifetimes, and stable performance. Light sources meeting these advanced inspection technique requirements do not exist. The lifetime, power, and stability of current DUV frequency converted lasers is generally limited by the frequency conversion crystals and conversion schemes, especially those exposed to DUV wavelengths like 355 nm, 266 nm, 213 nm, and 193 nm.
In spite of the challenges, advantages may be achieved when illuminating the wafer or reticle with light having wavelengths at or below 400 nm. However, providing suitable lasers for high quality wafer and photomask inspection systems is challenging. Conventional lasers generating light in the deep ultraviolet (DUV) range are typically large, expensive devices with relatively short lifetimes. Semiconductor wafer and photomask inspection systems generally require a laser having a high average power, low peak power, and relatively short wavelength to provide inspection having sufficient throughput and adequate defect signal-to-noise ratio (SNR).
High efficiency can be important for a DUV laser. High efficiency can allow a lower power fundamental laser source that can be more reliable, can be smaller, and can produce less heat. A low power fundamental laser can produce less spectral broadening if a fiber laser is used. Higher efficiency also tends to lead to lower cost and better stability. For these reasons, efficient frequency conversion to the DUV may be sought.
The primary method to provide adequate DUV power entails generating shorter wavelength light from longer wavelength light. This process of changing wavelengths is commonly called frequency conversion. Frequency conversion in this context uses high peak power density light to produce a nonlinear response in an optical crystal. To increase the efficiency of this process, the longer wavelength light may have high average powers, short optical pulses, and may be focused into the optical crystal. The original light is typically called fundamental light.
Relatively few nonlinear crystals are capable of efficient frequency conversion of light to UV/DUV wavelengths. Most crystals that have traditionally been employed have low damage thresholds if not properly prepared and if a tightly controlled operating environment is not maintained. Thus, the crystal has typically been contained within an enclosure to maintain the environment. To frequency convert an infrared laser to DUV, more than one crystal can be employed. When multiple crystals are employed, it can be advantageous to place them all within the enclosure. Crystal alignment complications can result, and it can be difficult to collect and focus light in such an enclosure.
Each successive node of semiconductor manufacturing requires detection of smaller defects and particles on the wafer. Therefore, higher power and shorter wavelength ultraviolet (UV) lasers for wafer inspection are needed. Because the defect or particle size is reduced, the fraction of the light reflected or scattered by that defect or particle is also typically reduced. As a result, an improved signal-to-noise ratio may be needed to detect smaller defects and particles. If a brighter light source is used to illuminate the defect or particle, then more photons will be scattered or reflected and the signal-to-noise ratio can be improved if other noise sources are controlled. Using shorter wavelengths can further improve the sensitivity to smaller defects because the fraction of light scattered by a particle smaller than the wavelength of light increases as the wavelength decreases.
Harmonic generation in nonlinear optical crystal materials is a technique to generate high power laser radiation in the visible, UV, and DUV spectral regions. Some inspection tools for wafers and reticle inspection used in the semiconductor industry rely on DUV radiation. Frequency doubling (i.e., second harmonic generation or “SHG”) is one commonly used form of harmonic generation. In order to achieve high conversion efficiencies, the phase velocities of the fundamental and the second harmonic waves may be identical (i.e., their phases are matched). This can be achieved in birefringent nonlinear crystals by adjusting the angle between the beam propagation direction and the optical axis “Z” of the crystal, as well as the temperature of the crystal. This phase matching condition can only be met when a suitable combination of phase matching angle and phase matching temperature is chosen. The required angle and temperature tolerances are typically in the range of tens of microradians and 0.1 K, respectively. Therefore, the nonlinear crystal is mounted on a base or inside an oven cell with a sufficiently accurate angle alignment and a sufficiently accurate temperature control.
Nonlinear crystals, commonly used for the generation of UV and DUV wavelengths include, but are not limited to, lithiumtriborate (LBO), beta-bariumborate (BBO), lithiumiodate, and cesium-lithiumborate (CLBO). With the exception of BBO, all of the above mentioned crystals are highly hygroscopic. Excess humidity can induce surface degradation in the case of LBO and CLBO. Excess humidity can destroy the entire crystal structure in the case of CLBO. Therefore, such crystals have to be either stored and operated in a sealed dry enclosure, or purged with a dry purge gas. Typical purge gases include, but are not limited to, clean dry air (CDA), argon, or nitrogen. One operating condition is to position the nonlinear crystal in a purged enclosure that is sealed except for the purge gas inlet and outlet. In addition to the dry purge, an elevated phase matching temperature, typically in the range of 50° C. to 200° C., is frequently chosen to provide additional protection against moisture.
When purging a heated oven cell, purge gas enters the oven cell at or near room temperature and mixes with the hot gas inside the chamber. The mixing of purge gases having different temperatures results in pointing variations and beam distortions of laser beams transmitted though the oven cell. If the nonlinear crystal is located inside a sealed and purged laser head with a typical length between 0.5 m and 1 m, the purge inlet can be located relatively far away from the crystal oven to minimize beam distortions and pointing variation. However, this may not be possible if a small, field replaceable, purged oven cell for the nonlinear crystal is used. In this case, the purged volume is small and the purge inlet is located close to the nonlinear crystal, which, as a consequence, is close to the laser beam transmitted through it. If CDA is used as a purge gas at an oven temperature of 100° C., the refractive index difference between the incoming air at room temperature and the air inside the oven at 100° C. is 6*10−6 at a wavelength of 300 nm. For an incoming air flow with 5 mm width along the beam propagation direction and a temperature gradient of 30° C./cm in the lateral direction, the result is a beam deflection of more than 10 micro-radians. A beam pointing variation on this order of magnitude can negatively impact applications that are sensitive to beam position and beam pointing, such as the generation of a flat-top beam profile using a diffractive optical element (DOE). If a turbulent purge-gas flow develops, temperature gradients are expected to be larger than is the example and, in addition, to show strong variations over time.
Therefore, what is needed is improved devices and operating techniques for nonlinear crystals or optical components, which minimizes the detrimental effects of a cold or room temperature purge gas flow entering a heated crystal enclosure.